1. Field of the Invention
The present invention relates to an endoscope system that measures an oxygen saturation level of blood, a processor device of the endoscope system, and an image processing method.
2. Description Related to the Prior Art
In a medical field, an endoscope system is widely used for diagnosis and treatment. The endoscope system is constituted of a light source device, an endoscope device, a processor device, a monitor device, and an input device. In use of the endoscope system, an insert section of the endoscope device is introduced into a patient's body cavity. An internal body portion is imaged by an image sensor provided at a distal end portion of the insert section, while being applied with illumination light from the distal end portion. There is known an endoscope system that obtains various types of living body information from an image captured using specific narrow band light as the illumination light.
According to an endoscope system of US Patent Application Publication No. 2012/0116192, for example, an oxygen saturation image, which images an oxygen saturation level of blood on a pixel-by-pixel basis, is produced with the use of narrow band light having a wavelength range in which an absorption coefficient is different between oxyhemoglobin and deoxyhemoglobin as the illumination light. In the oxygen saturation image, a hypoxic region representing a lesion such as cancer and a hyperoxic region representing a normal portion are displayed artificially with different colors. This facilitates grasping the distribution of the cancer at first sight.
To calculate the oxygen saturation level from the captured image, this endoscope system has the correlation among the oxygen saturation level and the intensity ratios of each individual pixel among a plurality of images captured under irradiation with a plurality of types of narrow band light, as data stored in advance in a memory such as a LUT. The oxygen saturation level is calculated using this correlation.
In this system, the correlation among the oxygen saturation level and the intensity ratios is obtained on the precondition that the difference in absorptance (absorption coefficient) between oxyhemoglobin and deoxyhemoglobin under irradiation with the narrow band light is invariable. For this reason, in the case of using a light source whose emission spectrum varies with time e.g. a xenon lamp, the difference in absorptance between oxyhemoglobin and deoxyhemoglobin deviates from a specified value with a lapse of time. Also, in the case of using a light source having a phosphor (for example, a phosphor absorbing blue excitation light and emitting green to red fluorescence) whose emission spectrum varies with temperature variation, the difference in absorptance between oxyhemoglobin and deoxyhemoglobin deviates from the specified value due to the temperature variation. Such deviation causes miscalculation of the oxygen saturation level.